Mr Thomas, On the assimilating tissues, etc. 413 



On the assimilating tissues of some Coal Measure Plants. By 

 H. Hamshaw Thomas, B.A., Downing College. 



[Read 7 February 1910.] 



The recent work in Fossil Botany has been concerned almost 

 entirely with morphological and phylogenetic considerations and 

 little attention has been given to the physiological significance 

 of the tissues studied. It is, however, very important from all 

 points of view to study so far as possible the biology and physi- 

 ology of the fossilised plants. This can be attempted by an 

 examination of our material from the standpoint of physiological 

 anatomy. The process of carbon assimilation is one of the most 

 important of the plant's vital activities and is closely connected 

 with leaf structure. Very little research has hitherto been done 

 on the subject of leaf structure in the Coal Measure period, and 

 at the suggestion of Mr Arber, the author is endeavouring to 

 obtain more information on this subject. 



The leaves of some members of the Galamocladus section of 

 the Calamites have been studied in detail. They are small linear 

 structures, almost cylindrical in cross-section. The simpler types 

 seldom exceed a length of 3 or 4 millimetres, and were about 

 •8 — 1 mm. broad. They were borne in whorls on long slender, 

 probably pendulous, stems. In the longer leaves a considerable 

 portion of the tissue is composed of thick-walled sclerenchymatous 

 elements which replace the mesophyll, forming a strand on the 

 adaxial side and increasing in proportions towards the apex of 

 the leaf. When this strengthening tissue is absent the leaf tissues 

 present a concentric arrangement, the central vascular bundle 

 being surrounded first by the bundle sheath and then by the 

 pallisade tissue. 



The mesophyll of the leaf is entirely composed of isolated 

 cylindrical cells, running perpendicularly out from the bundle 

 sheath to the epidermis, and forming a continuous pallisade tissue. 

 These cells are often completely separated from each other by 

 very large air spaces, and the tissue was consequently of an 

 extremely spongy character. Though this system of spaces would 

 provide an ample path for the circulation of carbon dioxide, 

 it would also probably bring about rapid transpiration. In the 

 larger leaves it is noticeable that the air spaces are considerably 

 reduced. There are indications that, as in the pallisade tissue of 

 modern leaves, the cells were specially modified to absorb a 

 maximum of the incident light. 



VOL. XV. PT. v. 27 



